It is known in the art to use U-nuts to fasten sheets of material, for example, sheet metal or sheet metal and plastic together as is shown, for example, in U.S. Pat. No. 3,426,818 to Derby issued on Feb. 1, 1969, and assigned to the assignee of the present application. There is shown in that patent a U-nut having a sleeve drawn from one portion of the U-nut in which threads are either machined or turned out by a tap in the form of ordinary screw-threads in which the thread angle comes to a point as in an ordinary screw. An example of this prior art is shown in FIG. 1, of the drawings in this application. Also discussed, but not illustrated, in the Derby patent is an example of the prior art shown in FIG. 2 of the present application in which only one thread, or a portion thereof, is formed in the portion of the U-nut containing the sleeve as shown in FIG. 1 of the present application, with, however, no sleeve actually being formed because the single thread does not require a collar.
By far the most commonly used U-nut in the art today is the one shown in FIG. 2 of the present application. This is due in part because of the inexpensive construction and in part because the sheet metal screws commonly in use today, for example in the automobile industry, have a thread shape in which the root of the thread is flattened rather than having the thread angles form a pointed thread at the apex of the thread angles.
Many such sheet metal screws and U-nuts are employed in, for example, the manufacture of an automobile. A problem which exists in the use of the U-nut shown in FIG. 2 is that overtorquing may cause the sheet metal screw to strip because all of the load is carried by one thread engaging the single thread portion of the prior art U-nut. Also, the U-nut itself may be damaged by overtorquing. Such damage to the screw or the U-nut may not be readily apparent but may ultimately result in failure of the screw or the U-nut, causing the panels which are held together thereby to become loosened. External forces, for example, due to vibration of the automobile in operation over time may result in this failure. Often, these U-nuts and screws are used during the assembly of the automobile and in the final assembly are in inaccessible locations in the automobile making replacement with a new screw and/or U-nut a difficult, time-consuming, or impossible task.
It is also desirable in many situations (e.g., automobile construction) to provide a nut for a sheet metal screw that has its cross-section deformed to an out-of-round condition after threading so that a screw threaded into the nut is tightly retained by the clamping effect of the nut. Such an arrangement is frequently referred to by the term "prevailing torque". The prior art nut of FIG. 2 cannot act in this way because only having a single thread leaves it prone to releasing the screw if the applied torque exceeds a specified limit.
The need exists, therefore, for a U-nut which is inexpensive to manufacture, multi-threaded, and adapted to receive the threads of a sheet metal screw having a flattened root on the thread angle. The present invention solves the problems of the prior art with a novel U-nut having a multi-threaded sleeve in which the threads are roll-tapped in a thread configuration to conform to a sheet metal screw having a flattened root. As a further and alternate aspect of the invention, it is contemplated to provide a U-nut for a sheet metal screw in which the U-nut has multiple threads along the nut sleeve for each screw thread.
The above description of the present invention has been summarized in order that the more detailed description which follows may be better understood and the contribution to the art better appreciated. The features of the present invention will be better understood by reference to the detailed description below and the accompanying drawing in which like-referenced numerals have been used to refer to like elements and wherein: